1. Field of the Invention
The present invention relates generally to a semiconductor structure and a process thereof, and more specifically to a semiconductor structure and a process thereof, that forms a silicon rich layer on a surface of a recess.
2. Description of the Prior Art
In the present semiconductor process, a localized oxidation isolation (LOCOS) or a shallow trench isolation (STI) are normally used to isolate the MOS transistors. However, with the reduction in both design size and fabricating line width of the semiconductor wafer, the drawbacks of pits, crystal defects and longer bird's beak in the LOCOS process will affect even more the characteristics of the semiconductor wafer. The field oxide produced in the LOCOS process also occupies a larger volume, which affects the integration of the semiconductor wafer. Thus, in the submicron semiconductor process, the STI process is widely used as an isolation technique because of its smaller size and improved integration potential.
The typical fabrication method of a STI is to first form shallow trenches between each MOS device in the surface of the semiconductor wafer, and a dielectric matter is filled into the shallow trenches to obtain an electrical isolation effect. The dielectric matter is composed of silicon oxide. As the silicon oxide is formed, the high temperature in steps for forming silicon oxide or latter processes would induce oxygen atoms diffusing into the silicon substrate of the active regions for forming transistors beside the recess, so that parts of the silicon substrate will be oxidized into silicon oxide. As a result, not only sizes of each shallow trench isolation structures can not be controlled precisely, but also the volumes of formed shallow trench isolation structures increase, which reduces the silicon substrate of the active regions. Even worse, as the sizes of the semiconductor components shrink and get close to their physical limitations, the shallow trench isolation structures with different sizes and the active regions with different sizes dramatically reversely affect the electrical performances of the components and their processing qualities.